1) Field of the Invention
The present invention relates to a multilayer optical recording medium having multilayer recording layers and an optical pickup device that records and replays the multilayer optical recording medium.
2) Description of the Related Art
In an optical recording medium such as a digital versatile disk (DVD), two layers of recording layer are formed, which allows for both a high recording density and a high capacity. To further increase the capacity of optical recording medium using an optical system of a higher NA (Numerical Aperture) and a light source of a short wavelength, a multilayer optical disk having three or four layers of recording layers has been proposed. In such an optical disk having several layers of recording layers, a spacer area is formed between a recording layer and another recording layer, and a semitransparent reflective film is formed on the entire surface of each recording layer.
Meanwhile, in recent years, to achieve still higher recording density and higher capacity in optical recording medium, the basic technology has been developed of a three-dimensional multilayer optical memory as a next generation optical recording medium having recording layers from around 10 layers to around 100 layers. As the recording layers of this three-dimensional multilayer optical memory, a photo polymer is employed that does not have a reflective film.
In the conventional multilayer optical memory, a structure is employed where a reflective film is not arranged on each recording layer, so as to reduce transmission loss of light for recording and reproducing at a maximum. Therefore, the reflectance ratio of each recording layer becomes the same as viewed from the side of a recording pickup. In addition, in this optical memory, to restrict as much as possible the influence of spherical aberration, the thickness is made extremely small of an intermediate layer (space area) that is formed between recording layers, i.e., interlayer space.
In the conventional optical memory with no reflective film in each recording layer, it is difficult to make a servo loop of a focus servo into a servo close condition at the side of a recording and reproducing device. Furthermore, because a reflective film is not formed on each recording layer and the interlayer space between recording layers is small, a focus error signal cannot be separated between layers, and it is difficult to make an interlayer jump that moves between layers (focus jump). Therefore, making a servo loop of a focus servo into a servo close condition and move control between layers are the issues that need consideration.